This invention relates to drop hammers in which a hammer weight is operated by a hydraulic pressure mechanism. Such hammers are known using a cylinder and piston combination in the hydraulic pressure mechanism, it usually being arranged that hydraulic fluid is admitted to the cylinder at one side of the piston to raise the hammer weight, and the subsequent descent of the weight is used as the working stroke to produce the hammer blow, e.g. to drive a foundation pile.
In such mechanisms, the maximum working rate is usually determined by the rate at which the hydraulic fluid can be fed to and released from the cylinder. The rise of the piston is obviously dependent upon the maximum rate at which pressure fluid can be fed to the cylinder but, in addition, during the fall of the hammer, when the cylinder space previously receiving pressure fluid is contracting, the maximum rate of flow from it may be limited by throttling effects causing a back pressure that slows the descent of the hammer weight, and the performance of the hammer will be correspondingly affected by this also.
UK Patent specification No. 1,261,220 describes a way of avoiding this difficulty by arranging that fluid is transferred between opposite sides of the piston at appropriate stages in a working cycle. In one arrangement disclosed, the piston itself contains the transfer valve, while in another arrangement a transfer valve is disposed externally of the cylinder, in a by-pass between inlet and outlet conduits connected to the cylinder. The first of these arrangements is particularly advantageous, since the fluid travels the shortest possible path between the spaces on opposite sides of the piston. The externally disposed valve arrangement has the advantage of being more easily accessible for servicing but is less efficient in operation.
It is found that although the form of transfer valve shown in this earlier patent can give good results, even the arrangement in which the transfer valve is contained in the piston can only be usefully employed up to a maximum size of hammer. If the scale of the design is further increased, inertia effects become more prominent and eventually interfere with the correct operation of the valve and also reduce undesirably its reliability. The alternative arrangement described, with the valves in a by-pass between inlet and outlet conduits to the hydraulic cylinder, is not subjected to these particular disadvantages but it is relatively inefficient and is equally unsatisfactory for larger hammers. The problem therefore remains of producing a large hydraulically driven drop hammer, e.g. with a hammer weight of 20 tonnes or more, that can be operated at a relatively high striking rate.